Genuine 3D movies — like Avatar, which returns to theaters today for a limited time — are shot with stereo cameras that create a ghost image brought into focus by 3D glasses. Fake 3D films use computers to draw over a 2D original.﻿

James Cameron knows a potential money grab when he sees it.

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Only eight months after igniting a 3D windfall at theaters worldwide with Avatar, Cameron is bringing back his billion-dollar baby — complete with nine minutes of additional footage, including that Na'vi alien sex scene the Avatards clamored for — for a limited time beginning today, just when it appears that moviegoers are losing interest in 3D.

Blame that on a combination of the extra fee for 3D glasses (typically $4) and movies billed as 3D that technically aren't. At least nine major releases so far in 2010 inflated their box office totals with that eyeglasses surcharge — but only five were created from scratch as 3D projects. The rest are movies filmed in traditional 2D then converted to a form of 3D that many viewers are calling inferior and not worth the extra ticket price. Avatar caught studios by surprise, and they're taking shortcuts to get into the 3D chase.

Genuine 3D movies (sometimes called "native 3D") like Avatar are shot with stereo cameras, with two lenses slightly offset, creating a ghost image that 3D glasses bring into focus. 2D movies use traditional single-lens cameras, and conversion to 3D is accomplished with computers "drawing" a reasonable facsimile of a ghost image. It's a digital lithograph, so to speak, rather than an original painting.

(In the case of computer-animated films, the digitized "ghosting" process is simpler, which partly explains why so many films in the current 3D trend are 'toons.)

Even when the process succeeds, the fact is that some converted movies weren't planned with in-your-face moments in mind. Reshoots or adding scenes is impractical. 3D conversion makes backgrounds appear deeper and richer (i.e. Alice in Wonderland), but if little optically extrusive effects happen in the foreground, the effect loses meaning (see: Clash of the Titans). Not to mention the extra $4 per ticket lost by moviegoers expecting an Avatar-like experience that wasn't technically possible in the first place.